The synthesis of linear triblock thermoplastic elastomers, such as PMMA-b-PIB-b-PMMA, is widely known. J. P. Kennedy, J. L. Price and K. Koshimura, "Novel Thermoplastic Elastomer Triblocks of Soft Polyisobutylene Mid-Block Connected to Two Hard Stereocomplex Outer-Blocks", Macromolecules, Vol. 24, p. 6567 (1991); J. P. Kennedy and J. L. Price, "Synthesis, Characterization and Physical Properties of Poly(methyl methacrylate-b-isobutylene-b-methyl methacrylate) Triblock Copolymers", Vandenberg Symposium, Polym. Mat. Sci. Eng. Vol. 64, 40-41 (1991); and J. P. Kennedy and J. L. Price, "Poly(methyl methacrylate)-block-Polyisobutylene-block-Poly(methyl methacrylate) Thermoplastic Elastomers: Synthesis, Characterization, and Some Mechanical Properties", ACS Symp. Ser. #496, pages 258-277 (1992).
It is generally recognized, however, that three arm star molecules exhibit more advantageous viscosity properties and mechanical properties than linear triblock molecules.
Recently, the synthesis of various multi-arm radial or star polymers has become of growing practical and theoretical interest to a variety of industries. Such star polymers are seen as useful as surfactants, lubricants, rheology modifiers, viscosity modifiers, adhesives and coatings. In fact, star polymers are now considered by many to be state-of-the-art viscosity modifiers and oil additives, although the potential of some of these star polymers for these applications is still being evaluated and tested.
For example, there has been a growing interest in star polymers consisting of multiple polyisobutylene (PIB) arms. Kennedy et al., U.S. Pat. No. 5,395,885 describes the synthesis of star polymers having multiple PIB arms and polydivinylbenzene (PDVB) cores using cationic, "arm-first", synthesis techniques. Because the structure of PIB contains no unsaturation, these PIB-based stars are believed to be useful for a variety of applications such as motor oil additives and viscosity index improvers.
In addition, star polymers, formed by the "core first" method, having multiple arms radiating from the core are also disclosed in the prior art. For example, U.S. Pat. No. 5,804,664 to Kennedy et al. discloses star polymers comprising a core component selected from the group consisting of a p-methoxy cumyl group and a calix[n]arene where n=4 to 16; and N number of arms containing at least one segment of polyisobutylene connected to said core component, where N=1 when said core component is said p-methoxy cumyl group and N=n, when said core component is said calix[n]arene. The star polymers of the this reference have only arm emanating from each aromatic ring.
It is desirable in the art to provide thermoplastic elastomer star compositions of matter having improved mechanical and thermal properties. While the preparation of star polymers is known in the art, heretofore, it has not been known to prepare three-arm star compositions of matter comprising an aromatic core and three diblock arms, each arm having inner and outer polymeric segments, wherein the outer segments of each diblock arm are obtained by living radical polymerization.